The core component of some conventional printed circuit boards (PCBs) is non-homogeneous. For example, the conventional FR4 material is formed of a weave of glass fiber bundles embedded in an epoxy resin. The non-homogeneity of the board material may have an adverse effect on signal propagation in a differential bus on the board, particularly in the case of data rates higher than about 1 Gb/s to 2 Gb/s. This adverse effect may be referred to as the “bundle weave effect”.
The bundle weave effect is a result of the difference in dielectric constant between the glass fiber material and the epoxy resin. Typically, the direction of a pair of traces for a differential bus may be parallel to one of the weave directions. Consequently, in some cases, one of the traces may overlie a glass fiber bundle along the length of the trace, while the other trace may overlie epoxy resin along the length of the trace (and disregarding the cross bundles in the weave). The difference in dielectric constant between the two materials causes the respective signals in the two traces to propagate at different propagation velocities, leading to phase skew and reduction or elimination of the signal eye width (also known as the timing window). As a result the transmitted signal may not be properly received. The bundle weave effect increases linearly with the length of the bus, and also increases linearly with data rate. Thus, the bundle weave effect can be expected to become of increasing concern as data rates increase.